Plk is the putative mammalian homologue of the Drosophila polo kinase and is implicated in regulating mitotic spindle formation and function. Plk kinase activity peaks during mitosis and Plk protein localizes at portions of the mitotic spindle and has been shown to associate with CHO1/klp, a kinesin-like motor protein. We have demonstrated that overexpression of wild type Plk transforms NIH3T3 cells in vitro and that such transformed cells form tumors in nude mice. Fluorescence microscopy indicates that cells transformed by Plk are highly aneuploid and frequently form abnormal mitotic spindles. Aberrant chromosome segregation caused by abnormal mitotic spindles may induce both aneuploidy and oncogenic transformation. Plk protein was found to be highly expressed in 16 of 57 human tumor cell lines and in several types of primary tumors. Deregulated expression of Plk may be involved in the origin or progression of some human cancers. Recently we have been involved in purifying Plk from cell extracts with the goal of identifying other proteins that interact with Plk. We have found that Plk is a member of a large complex and is associated with alpha, beta and gamma tubulins independent of microtubule polymerization state. We demonstrated that Plk association with tubulins is dependent on the kinase domain of Plk but that the kinase activity is irrelevant to the association. In light of the clear involvement of Plk in regulating the centrosomes and the mitotic spindle these results help provide a molecular explanation for the fole of Plk in cell cycle progression. More recently we have identified 10 other proteins that are found complexed with Plk and have determined the functional significance of these interactions. This work has been submitted for publication. An additional protein that associates with Plk has been identified. This protein appears to associate with Plk in a unique complex that does not contain the other identified proteins. Our current work is focused on determining what effect Plk mediated phosphorylation may play with this newly identified molecular partner.